The use of dwarf plants in the context of agricultural production has many advantages; for example, in cereals, the use of short-straw mutant plants has made it possible to produce crops which tolerate considerable amounts of nitrogen-containing fertilizers, which are less affected by weather conditions and, in particular, more resistant to torrential rain than the plants which are normal in size. In addition, the small size of the plants facilitates the maintenance of the crops, in particular the application of plant-protection treatments, and also the harvesting thereof.
Dwarf mutants of plants other than cereals have also been described in the literature. Mention will in particular be made below of mutants which have characteristics similar to those induced by a deficiency in gibberellins and which are insensitive to the providing of exogenous gibberellins. Such mutants have in particular been described in Arabidopsis [Koornneef et al., Physiol. Plant., 65, 33-39, (1985)]. These mutants, named gai (for gibberellic acid insensitive) are smaller in size and do not respond to exogenous applications of gibberellins. The gai mutation is a semi-dominant mutation of the “gain of function” type. GAI/gai heterozygous mutants have an intermediate phenotype between that of gai/gai dwarf mutants and GAI/gai wild-type plants.
Mutants having the same characteristics as the gai mutants of Arabidopsis have been described by Zanewich et al. [J. Plant Growth Regul., 10, 121-127, (1991)], in Brassica napus (dwf1 mutation) and Brassica rapa (mutations named dwf1 and dwf2).
The team of the inventors has obtained a dwarf mutant of B. rapa [Foisset et al., Theor. Appl. Genet., 91, 756-761, (1995)]. The mutation, named bzh, has characteristics of “semi-dominance” and of insensitivity to gibberellins, which are similar to those of the gai mutation.
A rapeseed line, named ISN1770, homozygous for the bzh mutant allele, has been the subject of a Certificat d'Obtention Végétale [Plant Variety Protection Certificate] filed on May 18, 1998, with the CPOV [French Plant Variety Protection Office] (11 rue Jean Nicaud, 75007 Paris) under the reference 10751. A rapeseed hybrid, named “Lutin” (B017), comprising in its genome the bzh mutant allele in heterozygous form was proposed for listing in the Catalogue Français des Obtentions Végétales [French Catalogue of Plant Varieties] on Jul. 31, 1999, under the reference 072426.
The GAI gene of Arabidopsis has recently been cloned and sequenced [Peng et al., Genes and development, 11, 3194-3205 (1997); PCT application WO 97/29123 in the name of John Innes Centre Innovations Ltd.]. This gene-encodes a 532 aa protein (GAI). The gai allele, which is responsible for dwarfism, contains a deletion of 51 base pairs in frame with the reading frame, which leads to the absence of 17 aas located close to the N-terminal end of the GAI protein. The GAI protein is involved in the perception of and response to gibberellins, and is thought to act, in wild-type plants, as a negative regulator of cellular elongation in the absence of gibberellins.
Comparison of the GAI sequence with that of the translation products of other known genes has made it possible to place it in the family named GRAS [Pysh et al., The Plant Journal, 18(1), 11-119, (1999)] or VHIID [Schumacher et al., P.N.A.S., 96, 1, 290-295, (1999)].
This family encompasses, besides GAI, the RGA [Silvestrone et al., Genetics, 146, 1087-1099, (1998)] and SCARECROW [Di Laurenzio et al., Cell, 86, 423-433, (1996)] genes of Arabidopsis, and also the tomato LS (lateral suppressor) gene [Shumacher et al., P.N.A.S., 96, 1, 290-295, (1999)]. At the current time, about twenty genes belonging to the GRAS family have been identified in Arabidopsis. 
The proteins which make the GRAS family have a very variable N-terminal portion and a very conserved C-terminal portion with five recognizable motifs, in particular the VHIID motif.
The biological functions of most of these proteins are not yet precisely known, but their role as transcription factors is strongly presumed. The investigations carried out on the 4 most thoroughly studied genes at the current time (SCR, GAI, RGA and LS), show that these genes encode transcription factors involved in controlling the perception of and the response to gibberellins, and indicate the probable importance of this family in controlling the morphogenesis and the development of higher plants.